1. Field of the Invention
The present invention relates to a solenoid valve, and more particularly, to a solenoid valve which is particularly effective for brake-pressure control of an anti-lock brake system (ABS) for vehicle use.
2. Description of the Related Art
In brake-pressure control according to an ABS, as shown in FIG. 3, when rotation of a tire shows a tendency to lock, hydraulic pressure in a wheel cylinder is reduced by opening a reducing solenoid valve 102, and when the rotation of the tire resumes, the hydraulic pressure in the wheel cylinder is increased in a step-like fashion by opening a pressure increasing solenoid valve 101 for a few milliseconds periodically responsive to a pulse driving signal. If the cross-sectional area of the pressure increasing solenoid valve 101 is large, since an amount of the pressure increase per pulse signal becomes large, hydraulic pulsation increases and the tire easily reaches a lock-up state. Conventionally, in order to prevent the above problems and to increase a suitable amount of the hydraulic pressure in the brake-pressure control, a control orifice inside the pressure increasing solenoid valve 101 has been provided.
However, the control orifice increases the passage resistance between a master cylinder (M/C) and wheel cylinder (W/C) during normal braking operations, and thereby it causes a problem in which a long time delay until the braking operations take effect is introduced and it lowers the quality of the brake feeling. In an attempt to provide a device solving the above problems, Japanese Unexamined Patent Publication No. 64-49779. teaches that during normal braking operations, a hydraulic pressure valve is maintained in a wide-open state by holding a movable member with a permanent magnetic holding device. During brake-pressure control by the ABS, the movable member is separated from the holding device by means of electromagnetic power. After that, when the brake fluid flows through an inflow area installed a globe-shaped sealing member, a suction effect is created on the sealing member. Furthermore, when the brake fluid flows through holes formed in a sealing support member, a pressure difference is created between upper and bottom surfaces of the movable member, and a suction effect is created on the bottom surface of the movable member. Due to the suction effects, the valve does not open completely but moves within a narrow range. Therefore, it is possible to improve controllability of the brake fluid pressure and prevent time delay of the normal braking operations.
However, the above device is required to provide the permanent magnetic holding device not to create the suction effect on the bottom surface of the movable member during the normal brake operations. Also, additional electromagnetic power is required to remove the movable member from the holding device. Therefore, use of the device makes construction of the solenoid valve complex and increases its size.